1. Field of the Invention
The present invention relates to a white balance control apparatus and control method therefor and, more particularly, to a white balance control apparatus which controls the white balance of an image on the basis of the luminance and color difference signals of the image, and a control method therefor.
The present invention also relates to an image sensing apparatus using the white balance control apparatus according to the present invention.
2. Description of the Related Art
Image sensing apparatuses using image sensors, such as a digital camera and digital video camera, have a white balance control function of adjusting the color tone of a sensed image. The white balance control corrects a pixel value using a white balance correction value so as to sense a white subject as a white.
The white balance control includes manual white balance control to obtain a white balance correction value by sensing a subject the user wants to sense as a white, and auto white balance control to obtain a white balance correction value by automatically detecting, from a sensed image, a portion regarded to be white.
To perform auto white balance control, a white subject is sensed in advance under a variety of light sources to compute the color-evaluation values of sensed images. The color-evaluation values are plotted in a predetermined color-space coordinate system to determine a white color region for each light source.
FIG. 5 is a graph showing an example of a white color region. The negative direction of the x-coordinate (Cx) in the Cx-Cy coordinate system represents a color-evaluation value obtained upon sensing a white subject under a high-color-temperature light source, and the positive direction represents a color-evaluation value obtained upon sensing a white subject under a low-color-temperature light source. The y-coordinate (Cy) represents the degree of the green (G) component of a light source. As the color-evaluation value becomes larger in the negative direction, the green component becomes larger. That is, when the light source is a fluorescent light, color-evaluation values are distributed in the negative direction.
When performing auto white balance control, an image is divided into a plurality of blocks, and a color-evaluation value is obtained for each block. A block having a color-evaluation value falling within a white color region as shown in FIG. 5 is determined to be white. The pixel values of blocks determined to be white among a plurality of blocks which form an image are integrated. White balance correction values which make, for example, the respective color components (R, G, and B) of the integrated values equal to each other are computed.
However, the color-evaluation value of the skin color region obtained by shooting under solar light, and that of a white subject shot under tungsten light are distributed in almost the same region. According to a conventional auto white balance control method, a skin color shot under solar light may be erroneously determined as a white under tungsten light and be whitely corrected.
Recently, image sensing apparatuses which use a face detection technique to perform auto exposure control of properly exposing a human face or perform auto focus detection control of focusing on a human face have been commercially available. In this situation, it is proposed to employ the face detection technique in white balance control as well. Japanese Patent Laid-Open No. 2003-189325 intends to solve the problem of erroneously detecting a skin color as a white by performing white balance control while excluding a face region detected by a face detection circuit from white pixel detection targets.
However, the white balance control method disclosed in Japanese Patent Laid-Open No. 2003-189325 does not consider a measure against a case wherein a face region is erroneously detected. When a region which is not a human face is erroneously detected as a face region, the white color region becomes excessively small, degrading the precision of obtained color temperature information and that of white balance control.